1. Field of the Invention
The present invention is directed to an LED (light emitting diode) assembly and to a method of manufacturing the LED assembly, and which is particularly adapted to address issues of color differences between different LEDs within the LED assembly.
2. Description of the Background Art
Traditional light sources are most commonly either incandescent or gas discharge. Each has advantages and disadvantages. Although inexpensive to manufacture, the traditional incandescent bulb suffers from two disadvantages. First, most of the input energy of traditional lighting is wasted as heat or infrared (non-visible) light; only a small amount of the input energy is transferred to visible light. Second, the lifetime of the incandescent bulb is limited and when failure occurs it is catastrophic. Traditional fluorescent bulbs have a longer life, but have significant performance variations across a range of temperatures. At some colder temperatures fluorescent bulbs do not function at all. Halogen light sources are a slight improvement in efficiency and lifetime over incandescent light sources for a marginal increase in cost.
Traditional sources of lighting can produce exact colors by filtering. The filtering process takes white lighting and removes all the light except the required light of the specified color and therefore further reduces the efficiency of the light source. Traditional lighting also is broadcast in all directions from the source, which may not be advantageous when the goal is to illuminate a small object. Lastly, traditional lighting has a non-linear relationship between brightness and input current. This non-linearity makes it difficult to dim the light source easily.
LEDs overcome many of the disadvantages of traditional lighting because of their significantly longer lifetime, higher efficiency, and ability to direct the light. The Mean Time Between Failures (MTBF) of typical incandescent light sources is in the order of 10,000 hours. The MTBF of LEDs is on the order of 1-10 million hours. Typically only 5% of the input energy is transferred to visible light for an incandescent light. Similarly, for LEDs about 15% of the input energy is transferred to visible light. The ratio of lumens of light output divided by the watts of input energy is another way to look at the efficiency. Traditional lighting has about 17 lumens/watt, whereas LED based (white) light sources are about 35 lumens/watt. The efficiency improvement equates to lower power consumption or higher light output for similar applied power. Generally, an individual LED produces a low level of light output that is insufficient for usage as a light source. Combining a number of LEDs into an assembly or array allows the array to be a reliable and cost effective replacement for traditional light sources.
When designed and fabricated, an array of LEDs in an assembly can be electrically interconnected in parallel, in series, or any combination thereof. Additionally, the LEDs in the assembly can be a single base color or many different colors. By combining several different colors into one assembly, a wide range of specified colors can be displayed by the light engine. These LED light engine assemblies are gaining widespread usage because of their ability to reduce electrical usage, improve maintenance costs, and allow dynamic, custom color projection.
LED assemblies are also rapidly replacing light bulbs in the Human Safety marketplace. Human Safety applications might include traffic lights, safety beacons on towers, warning lights at rail crossings, emergency egress lighting, aircraft runway lighting, and many more applications. In these applications LED light sources are gaining popularity for two reasons: (1) the increased reliability of LEDs, and (2) the reduced costs and difficulty of the repair and maintenance functions.
At the present time LED based light engines are in operation for Human Safety Applications in hundreds of thousands locations throughout the world.
LED lighting is also beneficial in architectural and theatrical applications. The benefit lies not only with the ability to produce an exact and repeatable light for changing moods and emotions but also with the ability to produce these colors dynamically and across a large number of light sources. This practice has been available in theatrical lighting for many years in various forms with tremendous improvement in digital color on demand in the relatively recent past. For architecture, the practical use of color remains limited largely due to the cumbersome use of theatrical grade fixtures in architectural applications. The promise of LED lighting is the ability to accomplish dynamic color in a more useful form factor and in real time for both theater and architectural applications.
A typical LED assembly includes a number of LEDs installed into a system, and typically all of the LEDs are a single base color. The technology is progressing and new requirements are emerging for the production of a broad spectrum of colors from combinations of two, three, four or more base colors of LEDs. Many assemblies under development include several Red LEDs, several Green LEDs, and several Blue LEDs. Several LEDs are needed of each color, because a single LED does not provide sufficient light for a light engine. Different LED colors are needed so that the different colors can be combined to make a broad spectrum of custom lighting effects.
A generalized LED assembly 10 is shown in FIG. 1. The LED assembly 10 includes an LED light source 11, which in turn includes individual LEDs 12 of different colors represented by the designators—R (red), G (green), and B (blue). The LED assembly 11 includes the LEDs 12 and a support and associated circuitry for driving the LEDs. The associated circuit and support includes an electronic carrier or printed circuit board (not shown) to mechanically hold the LEDs 12 and to provide electrical input to the LEDs 12, a power supply 13 to convert input power into a usable form for the LEDs 12, control electronics 14 to turn the LEDs 12 on and off appropriately, perform algorithms on the electronic signal and communicate with other equipment in a larger lighting system, and a lens or diffuser (not shown) to modify the light appearance from several small point sources to a look that is both pleasing to a human and functional for the product.
LED assemblies do, however, have the following disadvantages recognized by the present inventor. Variations within manufacturing of the optical and electrical output properties are sizeable. Targeted output colors are difficult to achieve because of the manufacturing variations of the LEDs. The optical output varies over the product lifetime; for instance, the output intensity degrades with time. The dominant wavelength is highly dependent on temperature. And, intensity drops with temperature increases.
Further, for LEDs different semiconductor compounds are used to produce different colors. Each compound will change at a different rate with respect to temperature and long term degradation. This has made the color stability of an array of RGB (Red, Green, Blue) LEDs difficult.
The fact that LED light output varies proportionately with input current is generally an advantage of LEDs; it becomes a disadvantage when an LED assembly is used as a direct replacement for an incandescent bulb. This is because the control system compensates for the non-linearity of the incandescent bulb and produces nonsensical output with the replacement LED assembly.
Lighting control systems or consoles address a limited number of light outputs with a limited number of possible color specifications and may require cumbersome hardware to address large lighting systems.
Temperature variations of the LEDs can occur for two reasons. One source is the outside environment. LED light sources can be installed in controlled temperature environments, examples of which would be home or office buildings. Alternatively, they can be installed in uncontrolled temperature environments where temperature variations are in the range of human habitability and beyond. The second source of temperature variability is the efficacy of the thermal dissipation within the specific system. Optical output properties are related to the die temperature. The die temperature is related to the outside environment, but also the thermal resistance of the entire path from the die to the outside world.
The dominant wavelength (represented by λd) and the optical intensity exhibit quantifiable changes with these temperature changes. With sufficient temperature variations the change in the dominant wavelength can be discernible by the human eye. At some wavelengths (near the color amber) changes of 2-3 nanometers (nm) are discernible to the human eye; at other wavelengths (near the color red) changes of 20-25 nm are required before the human eye can differentiate a color shift. The intensity change with temperature is discernible as well. Temperature increases of 60° C. can reduce output by approximately 50%.
The current state of the art partially addresses the issues. The manufacturing variation of the LED optical output is resolved by sorting or binning the LEDs into groupings of similar optical properties. The optical response of an incandescent light has been mimicked in the control software and hardware for the array, see for example U.S. Pat. No. 6,683,419. The initial power output of the LED can also be over-driven, which results in acceptable power outputs over a longer period of time.
The current state of the art, however, does not resolve the following issues. Exact color generation of a specified color is still not achievable. Binning of the LEDs is not always sufficient to produce an accurate color across all environments because of the wide variations in the LED optical properties within a bin. Temperature variations effects on LED output wavelength and intensity are not compensated for.